One of the many bugs that take their time to get a fix, if they ever will.
In the mean time, try and resolve your issue through the option given by Andaro in the thread Pim linked to.
Or, for lazy people:
Place the meldable items you are having trouble with in the first row of your inventory.
Please file a support ticket via Contact Us
Game Support > Last Chaos > Other
Subject Line "Melding Recovery"
Message "Hi, I would like to have my [insert costume name] reset so I may meld/unmeld again. Thanks so much for all that you do, signed [your name]"
Please allow several days for your request to be completed.
GM's are very busy, but will get to you in a timely fashion, in the order your RT was received.
It's a workaround that supposedly works, worth giving it a shot.